Composite carbon molecular sieve membranes having anti-substructure collapse particles loaded in a core thereof

ABSTRACT

A carbon molecular sieve (CMS) membrane is made by pyrolyzing, to a peak pyrolysis temperature TP, a hollow fiber membrane having a polymeric sheath surrounding a polymeric core, anti-substructure collapse particles present in pores formed in the polymeric core help prevent collapse of pores formed in the hollow fiber membrane before pyrolysis. The anti-substructure collapse particles are made of a material or materials that either: i) have a glass transition temperature TG higher than TP, ii) have a melting point higher than TP, or ii) are completely thermally decomposed during said pyrolysis step at a temperature less than TP. The anti-substructure collapse particles are not soluble in a solvent used for dissolution of the polymeric material of the core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/827,064, filed Aug. 14, 2015, which claims the benefit of priorityunder 35 U.S.C. § 119 (e) to U.S. Provisional Patent Application No.62/085,625, filed Nov. 30, 2014, the entire contents of which areincorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to carbon molecular sieve membranes andgas separations utilizing the same.

Related Art

Membranes are often preferred to other gas separation techniques inindustry due to the following advantages. The energy consumption formembranes is low as they do not require a phase change for separation.Membrane modules are compact, thereby reducing their footprint andcapital cost. Membranes are also mechanically robust and reliablebecause they have no moving parts.

Polymer membranes in particular are used in a wide variety of industrialapplications. They enable the production of enriched nitrogen from air.They separate hydrogen from other gases in refineries. They are alsoused to remove carbon dioxide from natural gas.

However, owing to the manufacturing processes and material structure,today's polymeric membranes cannot reach both high selectivities andpermeabilities, because a trade-off exists between permeability andselectivity. Robeson formulated semi-empirical upper-bound trade-offlines for several gas pairs. (Robeson, “The upper bound revisited”,Journal of Membrane Science 2008, vol 320, pp 390-400 (2008)). Carbonmembranes exceed this upper-bound and therefore are quite promising.

Since the production of crack-free, hollow fiber, carbon molecular sievemembranes (CMS membranes) in the late 80s, researchers have shown thatthese carbon membranes offer several advantages over polymericmembranes. They have better intrinsic properties and exhibit betterthermal and chemical stability. Thus, they have minimal plasticizationaffects.

CMS membranes are produced by pyrolyzing polymeric precursor membranes(i.e., green membranes) at temperatures of about 400-700° C. in acontrolled atmosphere. In regards to controlled atmosphere, US2011100211A discloses the importance of oxygen doping during pyrolysisprocess. It claims that oxygen doping can be tuned in order to obtainthe desired properties for the CMS membrane.

The properties of CMS membranes also depend upon the choice of precursorpolymer. Various polymer precursors are disclosed in the non-patent asbeing suitable for formation of CMS membranes. U.S. Pat. No. 6,565,631discloses the use of Matrimid and 6FDA/BPDA-DAM. U.S. Pat. No. 7,947,114discloses the use of cellulose acetate polymer. US 2010/0212503discloses the use of polyphenylene oxide (PPO).

While the above disclosures have shown that the CMS membrane materialshave superior intrinsic characteristics compared to those of precursorpolymeric materials, there still exists a challenge of making high fluxCMS hollow fiber membranes. This challenge is related to the fibersubstructure morphology. In hollow fiber membrane spinning, acomposition including polymer and solvent (aka the dope solution) and abore fluid are extruded from a spinneret. The bore is extruded from acircular conduit while the dope solution is extruded from an annulusdirectly surrounding the bore fluid.

The dope solution composition can be described in terms of a ternaryphase diagram as shown in FIG. 1. The polymer loading and amounts ofsolvent and non-solvent are carefully controlled in order to produce asingle phase that is close to binodal. That way, as the extruded borefluid and dope solution exit the spinneret and traverse through an airgap, solvent evaporating from the dope solution causes the exterior ofthe dope solution to solidify, thereby forming an ultrathin, dense skinlayer. As the nascent fiber is plunged into a coagulant bath containingnon-solvent, exchange of solvent and non-solvent from the fiber to thebath and vice-versa causes the remaining, inner portion of thenow-solidifying fiber to form a two-phase sub-structure of solid polymerand liquid solvent/non-solvent.

After drying to remove remaining amounts of the solvent and non-solvent,the spaces in the sub-structure formerly containing solvent andnon-solvent are left as an interconnecting network of pores within thatsub-structure that contribute towards high flux. The final result is anasymmetric green fiber comprising a thin, dense skin over a thick, lessdense, porous sub-structure.

During pyrolysis process, the pore network in the substructure collapsesand densifies with the result of producing an effectively much thickerdense skin layer. Since flux is dense skin layer-dependent, a very thickdense skin can significantly decrease the flux exhibited by the CMSmembrane. While the use of higher glass transition temperature (Tg)polymers in the dope solution may lower the relative degree ofsubstructure pore collapse, suitably high fluxes are predicted to remainelusive without a solution to the foregoing problem.

Researchers have come up with two different approaches for making highflux CMS hollow fiber membranes.

One approach is to form a thin walled fiber. Since essentially theentire fiber wall collapses during pyrolysis to form an effectively muchthicker dense skin layer, the obvious method of increasing the permeanceof a hollow fiber membrane is to decrease its overall wall thickness.The drawback of this method is that, as fiber wall thickness is reduced,the strength of the resultant CMS membrane is compromised.

Therefore, it is an object of the invention to provide a CMS membrane(and method making the same) having a relatively high flux that exhibitsa satisfactory degree of strength.

Another approach is to form silica structures within the CMS membrane.US 20130152793 discloses the immersion of precursor hollow fibers invinyl-trimethoxy silane (VTMS) for about 1 day, withdrawing them fromthe VTMS, and allowing them to remain in an ambient air environment forabout another day. After removal, the VTMS impregnated in the fiberreacts with moisture in the air to form a silica structure in the poresof the fiber substructure. This silica structure prevents those poresfrom collapsing during the subsequent pyrolysis. While this approachhelps improve the CMS membrane flux, it does require an additionallengthy processing step (immersion within VTMS) above and beyondconventional techniques. An additional processing step creates abottleneck to the overall production process that was not previouslypresent with conventional techniques. An additional processing step alsointroduces another opportunity for poorly controlled variables to leadto non-uniform CMS membranes over time. An additional processing stepalso increases the footprint of the manufacturing process. Moreover,VTMS is a flammable liquid requiring careful handling. As a result ofthe foregoing issues, from a cost, complexity, throughput rate,manufacturing uniformity, manufacturing footprint, and safety point ofview, the approach advocated by US 20130152793 is not fullysatisfactory.

Therefore it is another object of the invention to provide a CMSmembrane (and method of making the same) that does not require anadditional processing step beyond conventional techniques, which isrelatively less expensive, which is less complex, which does not pose abottleneck to an overall throughput of manufacture, and which isrelatively more safe than the solution proposed by US 20130152793.

SUMMARY

There is disclosed method for producing a CMS membrane that comprisesthe following steps. Composite precursor polymeric hollow fibers areformed, each having a sheath covering a hollow core, the core comprisinga polymeric material and silica particles. The composite precursorpolymeric hollow fibers are pyrolyzed.

There is also disclosed a method for separating a gas mixture thatcomprises the following steps. The gas mixture is fed to the CMSmembrane made according to the above-disclosed method. A permeate gas iswithdrawn from one side of the CMS membrane that is enriched in at leastone gas relative to the gas mixture. A non-permeate gas is withdrawnfrom an opposite side of the CMS membrane that is deficient in said atleast one gas relative to the gas mixture.

There is also disclosed a method for producing a CMS membrane fiber,comprising the following steps. A composite precursor polymeric hollowfiber is formed having a sheath covering a hollow core, the core beingsolidified from a core composition comprising a polymeric core materialdissolved in a core solvent and anti-substructure collapse particlesinsoluble in the core solvent, the anti-substructure collapse particlesbeing disposed within pores formed in the polymeric core material, thesheath being solidified from a sheath composition comprising a polymericsheath material dissolved in a sheath solvent, the anti-substructurecollapse particles having an average size of less than one micron. Thecomposite precursor polymeric hollow fiber is pyrolyzed up to a peakpyrolysis temperature T_(P). The anti-substructure collapse particlesare made of a material or materials that either: i) have a glasstransition temperature T_(G) higher than T_(P), ii) have a melting pointhigher than T_(P), or ii) are completely thermally decomposed duringsaid pyrolysis step at a temperature less than T_(P).

There is also disclosed a CMS membrane module including a plurality ofthe above-disclosed CMS membrane fibers.

There is also disclosed a method for separating a gas mixture,comprising the steps of feeding a gas mixture to the above-disclosed CMSmembrane module, withdrawing a permeate gas from the CMS membrane modulethat is enriched in at least one gas relative to the gas mixture, andwithdrawing a non-permeate gas from the CMS membrane module that isdeficient in said at least one gas relative to the gas mixture.

Any of the methods, resultant CMS membrane, CMS membrane fiber, or CMSmembrane module may include one or more of the following aspects:

-   -   the sheath does not contain silica particles.    -   the silica particles are of submicron particle size.    -   the polymer of the core and sheath is 6FDA:BPDA/DAM.    -   the core does not contain Matrimid.    -   the core contains less than 100% Matrimid.    -   the sheath does not contain Matrimid.    -   the sheath contains less than 20% Matrimid.    -   the material or materials of the anti-substructure collapse        particles are selected from the group consisting of: polymerics,        glasses, ceramics, graphite, and mixtures of two or more        thereof.    -   the material of the anti-substructure collapse particles is        polybenzimidazole.    -   the material of the anti-substructure collapse particles is        silica.    -   the material or materials of the anti-substructure collapse        particles are selected from cellulosic materials and        polyethylene.    -   the polymeric sheath material and the polymeric core material        are a same polymer or copolymer.    -   a wt % of the polymer or copolymer in the core composition is        lower than a wt % of the polymer or copolymer in the sheath        composition.    -   the polymeric sheath material is different from the polymeric        core material.    -   the polymeric sheath material comprises a major amount of a        first polymer or copolymer and a minor amount of second polymer        or copolymer and the polymeric core material comprises a minor        amount of the first polymer or copolymer and a major amount of        the second polymer or copolymer.    -   the polymeric sheath material is a first polymer having a first        coefficient of thermal expansion, the polymeric core material is        a second polymer having a second coefficient of thermal        expansion, and the first and second coefficients of thermal        expansion differ from one another by no more than 15%.    -   the first coefficient of thermal expansion is greater than the        second coefficient of thermal expansion.    -   a wt % of the anti-substructure collapse particles in the core        composition is selected such that the polymeric sheath material        shrinks along a length of the fiber no more than +1-15% than        that of the polymeric core material, but in any case is at least        5 wt %.    -   the polymeric sheath material is a first polymer exhibiting a        first coefficient of thermal shrinkage above a temperature at        which the first polymer starts to thermally degrade, the        polymeric core material is a second polymer having a second        coefficient of thermal shrinkage above a temperature at which        the second polymer starts to thermally degrade, and the first        and second coefficients of thermal shrinkage differ from one        another by no more than 15%.    -   the polymeric sheath material is a first polymer, the polymeric        core material is a second polymer, and the second polymer has a        glass transition temperature equal to or greater than 200° C.    -   the second polymer has a glass transition temperature equal to        or greater than 280° C.    -   the polymeric core material is        poly(meta-phenyleneisophthalamide).    -   the polymeric core material is the condensation product of        2,2-bis[4-(2,3-dicarboxyphenoxy)phenyl]propane dianhydride and        m-phenylenediamine or p-phenylenediamine.    -   the polymeric core material is polybenzimidazole    -   each of the polymeric core and sheath materials is made of a        polymer or copolymer selected from the group consisting of        polyimides, polyether imides, polyamide imides, cellulose        acetate, polyphenylene oxide, polyacrylonitrile, and        combinations of two or more thereof.    -   the polyimide is 6FDA:BPDA/DAM.    -   the polyimide consists of the repeating units of formula I:

-   -   the polyimide is selected from the group consisting of:        6FDA:mPDA/DABA and 6FDA:DETDA/DABA.    -   the polymeric sheath material is poly        (4,4′-oxydiphenylene-pyromellitimide).    -   the polymeric sheath material consists of the repeating units of        formulae II and III:

-   -   the polyimide consists of repeating units of formula IV:

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 is an illustrative phase diagram of for mixtures of polymer,solvent and non-solvent.

FIG. 2 is a SEM image of a CMS membrane fiber that lacks silicaparticles.

FIG. 3 is a SEM image of a CMS membrane fiber that includes silicaparticles in the core.

DESCRIPTION OF PREFERRED EMBODIMENTS

During pyrolysis of the precursor hollow fiber, effectively thick densefilms (that are caused by collapse of the fiber wall) may be preventedby forming the precursor hollow fiber with a composite morphology thatincludes a sheath covering a core that comprises a polymer core materialand sub-micron size anti-substructure collapse particles.

The anti-substructure collapse particles are made of materials that mayact in one of two ways. They thermally decompose substantiallycompletely during pyrolysis, at a temperature less than a peak pyrolysistemperature, to yield a porous core in the final CMS membrane fiberproduct that exhibits high flux. Alternatively, they do not form poresthemselves during pyrolysis and do not flow or melt at the peaktemperature during pyrolysis. Rather, they prevent the pores that arealready present in the precursor hollow fiber from collapsing duringpyrolysis so as to yield a porous core in the final CMS membrane fiberproduct that also exhibits high flux.

In the absence of the anti-substructure collapse particles of theinvention, the pores that are present in the core of the precursorhollow fiber would collapse during pyrolysis and the core would densifyso as to yield a relatively non-porous core in the final CMS membranefiber, and more importantly, an effectively much thicker dense layerthat prevents relatively high flux through the membrane.

The first type of material used for the anti-substructure collapseparticles include organic materials such as cellulosic materials andpolyethylene.

The second type of material used for the anti-substructure collapseparticles include polymers or copolymers that have a glass transitiontemperature, T_(G), higher than a peak temperature reached duringpyrolysis. These polymers or copolymers are insoluble in the solventused to dissolve the polymeric core material, because if they did infact dissolve in that solvent, they would no longer result in solidparticles inhibiting collapse of the pores in the core during pyrolysis.Particularly suitable polymers or copolymers have a T_(G) that is atleast 50° C. higher than the peak pyrolysis temperature. One example ispolybenzimidazole (PBI).

The second type of material used for the anti-substructure collapseparticles also includes inorganic materials that have a melting pointhigher than the peak pyrolysis temperature. Particularly suitableinorganic materials have a melting point that is at least 50° C. higherthan the peak pyrolysis temperature. Examples include glasses (such asparticulate fiberglass), ceramics (such as ZiO₂, TiO₂, perovskites,zeolites, and silica. Suitable silica particles may be obtained fromSpectrum Chemical Corp. under the trade name Cab-O-Sil M-5.

Regardless of which type of anti-substructure collapse particle materialis used, because the core is not densified to the degree of theultrathin dense film in practice of the invention, the flux of permeatethrough the membrane far exceeds conventional CMS membranes that do notincorporate a solution to the problem of pore collapse in the core.Those skilled in the art will recognize that the pyrolyzed polymericsheath material is densified and primarily responsible for separation offluids. The dense film formed from pyrolysis of the sheath is quite thin(often as little as 0.01 and as much as 5 microns, but typically 0.01 to0.10 microns) so as to not reduce the overall flux through the membrane.The skilled artisan will recognize that thinner dense films yield higherfluxes and thicker dense films yield lower fluxes. On the other hand,the much thicker pyrolyzed core material is relatively porous andpresents little resistance to permeation of fluids through the membraneas a whole and thus exhibits high flux.

The composite CMS hollow fiber membrane may be made by either of twogeneral methods. First, it may be made by co-extrusion of the core andsheath in the shape of a hollow fiber, phase inversion/coagulation ofthe nascent hollow fiber, and pyrolysis of the coagulated fiber. Second,it may be made by extrusion of the core in the shape of a hollow fiber,coagulation of the nascent hollow fiber, coating of the coagulatedhollow fiber with the polymeric material of the sheath, and pyrolysis ofthe coated fiber.

In the first general method, two different compositions (dope solutions)are prepared. The core dope solution comprises the polymeric corematerial dissolved in a solvent and the anti-substructure collapseparticles uniformly mixed in the polymer solution. The sheath dopesolution comprises the polymeric sheath material dissolved in a solvent.A typical procedure is broadly outlined as follows. A bore fluid is fedthrough an inner annular channel of spinneret designed to form acylindrical fluid stream positioned concentrically within the fibersduring extrusion of the fibers. A number of different designs for hollowfiber extrusion spinnerets known in the art may be used. Suitableembodiments of hollow-fiber spinneret designs are disclosed in U.S. Pat.No. 4,127,625 and U.S. Pat. No. 5,799,960, the entire disclosures ofwhich are hereby incorporated by reference. The bore fluid is preferablyone of the solvents (for example, NMP) described above for use in thecore or sheath dope solutions, but a mixture of water and a solvent maybe used as well. The core dope solution is fed through an intermediateannular channel of the spinneret surrounding the bore fluid and thesheath dope solution is fed through an outer annular channel of thespinneret surrounding the fed core dope solution. A nascent compositehollow fiber is obtained from the extrusion through the spinneret of thefed bore fluid and core and sheath dope solutions.

With continued reference to the first general method, the diameter ofthe eventual solid polymeric precursor fiber is partly a function of thesize of the hollow fiber spinnerets. The outside diameter of thespinneret annulus from which the core dope solution is extruded can befrom about 400 μm to about 2000 μm, with a bore solution capillary-pinoutside diameter from 200 μm to 1000 μm. The inside diameter of the boresolution capillary is determined by the manufacturing limits for thespecific outside diameter of the pin. The temperature of the core andsheath dope solutions during delivery to the spinneret and duringspinning of the hollow fiber depends on various factors including thedesired viscosity of the dispersion within the spinneret and the desiredfiber properties. At higher temperature, viscosity of the dispersionwill be lower, which may facilitate extrusion. At higher spinnerettemperatures, solvent evaporation from the surface of the nascent fiberwill be higher, which will impact the degree of asymmetry or anisotropyof the fiber wall. In general, the temperature is adjusted in order toobtain the desired viscosity of the dispersion and the desired degree ofasymmetry of the fiber wall. Typically, the temperature is from about20° C. to about 100° C., preferably from about 40° C. to about 80° C.

Upon extrusion from the spinneret, the nascent polymeric hollow fiber ispassed through an air gap and immersed in a suitable liquid coagulantbath. In the air gap, an amount of the solvent from the extruded sheathdope solution evaporates and a solid polymeric skin layer is formed. Inother words, the dissolved polymeric sheath material solidifies into askin layer. The liquid coagulant bath facilitates phase inversion of thedissolved polymeric core and sheath materials and solidification of theremaining portions of the precursor composite membrane structure. Thecoagulant constitutes a non-solvent or a poor solvent for the polymericmaterial(s) while at the same time a good solvent for the solvent(s)within the core and dope solutions. As a result, exchange of solvent andnon-solvent from the fiber to the bath and vice-versa causes theremaining, inner portion of the nascent fiber (i.e., substantially thecore) to form a two-phase substructure of solid polymer and liquidsolvent/non-solvent as it is drawn through the liquid coagulant bath.Suitable liquid coagulants include water (with or without awater-soluble salt) and/or alcohol with or without other organicsolvents. Typically, the liquid coagulant is water.

With continued reference to the first general method, theconcentration(s) of the polymeric material(s) and the relative amountsof the solvent(s) and non-solvent are selected so as to produce singlephases in the core and dope solutions that are close to binodal. Thatway, as the extruded bore fluid and core and sheath dope solutions exitthe spinneret and traverse through an air gap, solvent evaporating fromthe sheath dope solution causes the exterior of the extruded sheath dopesolution to vitrify, thereby forming an ultrathin, dense skin layer. Thetwo-phase substructure of the remaining portions of the nascentcomposite fiber (i.e., substantially the core) includes a matrix ofpolymer and pores that are filled with silica particles, solvent(s) andnon-solvent.

Typically, the solidified fiber is then withdrawn from the liquidcoagulant bath and wound onto a rotating take-up roll, drum, spool,bobbin or other suitable conventional collection device. An aspect ofthe extruding, immersing, and winding steps includes controlling theratio of solidified fiber windup rate to nascent fiber extrusion rate.This ratio is also sometimes called “draw ratio”. One of ordinary skillin the art will recognize that the combination of spinneret dimensionsand draw ratio serve to control the precursor fiber dimensions to thedesired specifications.

Before or after collection, the fiber is optionally washed to remove anyresidual solvent(s) and non-solvent. After collection, the fiber isdried in order to remove any remaining solvent(s) or non-solvent). Afterthe drying and optional washing steps, the pores that formerlycontaining solvent and non-solvent remain filled with the silicaparticles. Thus, an asymmetric, composite hollow precursor fiber isformed that comprises an ultrathin, dense skin over a thick coreincluding silica particle-filled pores.

In the second general method, a core dope solution and a sheath coatingsolution are prepared. The core dope solution comprises the polymericcore material dissolved in a solvent and the anti-substructure collapseparticles uniformly mixed in the polymer solution. The sheath coatingsolution comprises the polymeric sheath material dissolved in a solvent.The second general method is similar in many ways to the first generalmethod with the following exceptions. Instead of being co-extruded withthe core dope solution from a spinneret, the sheath coating compositionis coated onto the coagulated hollow fiber (with optional processingsteps known in the art of membrane manufacturing in between coagulationand coating for enhancing the achievement of a robust, uniform coating).

Regardless of whether the first or second general method is employed,the completed precursor composite hollow fibers have an outer diameterthat typically ranges from about 150-550 μm (optionally 200-300 μm) andan inner diameter that typically ranges from 75-275 μm (optionally100-150 μm). In some cases unusually thin walls (for example,thicknesses less than 30 μm) may be desirable to maximize productivitywhile maintaining desirable durability. The desired final thickness ofthe CMS membrane sheath layer (after extrusion, drawing, and pyrolysis)can be achieved by selection of appropriate spinneret dimensions (as thecase may be), coating conditions (as the case may be), draw ratios, andpyrolysis conditions to later result in sheath thicknesses as thin as0.01-0.10 microns. The desired final thickness of the CMS membrane corelayer can similarly be achieved through selection of appropriate valuesfor the corresponding conditions.

As mentioned above, the polymeric sheath material is primarilyresponsible for the separation of the fluids (i.e., gases, vapors and/orliquids) and is selected based upon separation performance. Thepolymeric sheath material may be any polymer or copolymer known in thefield of polymeric membranes for fluid separation and includes, but isnot limited to, polyimides, polyether imides, polyamide imides,cellulose acetate, polyphenylene oxide, polyacrylonitrile, andcombinations of two or more thereof.

Typical polyimides include 6FDA:BPDA/DAM, 6FDA/mPDA:DABA,6FDA/DETDA:DABA, Matrimid, Kapton, and P84. 6FDA:BPDA/DAM, shown below,is a polyimide synthesized by thermal imidization from three monomers:2,4,6-trimethyl-1,3-phenylene diamine (DAM),2,2′-bis(3,4-dicarboxyphenyl hexafluoropropane) (6FDA), and3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride (BPDA).6FDA:BPDA/DAM is a polyimide made up repeating units of 6FDA/DAM andBPDA/DAM:

6FDA/mPDA:DABA is a polyimide synthesized by thermal imidization fromthree monomers: 2,2′-bis(3,4-dicarboxyphenyl hexafluoropropane) (6FDA),1,3-phenylenediamine (mPDA), and 3,5-diaminobenzoic acid (DABA).6FDA/DETDA:DABA is a polyimide synthesized by thermal imidization fromthree monomers: 2,2′-bis(3,4-dicarboxyphenyl hexafluoropropane) (6FDA),2,5-diethyl-6-methyl-1,3-diamino benzene (DETDA), and 3,5-diaminobenzoicacid (DABA). Matrimid has the repeating units of formula I:

Kapton is poly (4,4′-oxydiphenylene-pyromellitimide). P84 consists ofrepeating units of formula IV:

A suitable polyether imide includes Ultem having the repeating units offormula C:

A suitable polyamide imide includes Torlon having the repeating units offormulae D and E:

The polymeric core and sheath material(s) may be the same or different.They typically has a relatively higher glass transition temperature (Tg)in order to reduce the degree to which non-silica-filled pores in thecore collapse, assuming that the membrane does not spend relatively muchtime above its Tg during pyrolysis. One such polymer is 6FDA:BPDA/DAM.

In order to inhibit delamination of non-identical sheath and corepolymeric materials during pyrolysis, a portion of the polymeric sheathmaterial may include a major amount (e.g., greater than 50 wt % and asmuch as 99 wt %) of a first polymer or copolymer and a minor amount(e.g., less than 50 wt % and as little as 1 wt %) of second polymer orcopolymer. Similarly, the polymeric core material comprises a minoramount (e.g., less than 50 wt % and as little as 1 wt %) of the firstpolymer or copolymer and a major amount (e.g., greater than 50 wt % andas much as 99 wt %) of the second polymer or copolymer. By blending inan amount of each polymer in each layer, the affinity of the core forthe sheath may be enhanced.

As another technique for inhibiting delamination of non-identical sheathand core polymeric materials during pyrolysis, the polymeric sheathmaterial is a first polymer having a first coefficient of thermalexpansion, the polymeric core material is a second polymer having asecond coefficient of thermal expansion, and the first and secondcoefficients of thermal expansion differ from one another by no morethan 15%. By at least roughly matching the coefficients of thermalexpansion, unevenness in the linear expansion along the length of thefiber during the heating of the pyrolysis step may be reduced orvirtually eliminated. Typically, the first coefficient of thermalexpansion is greater than the second coefficient of thermal expansionsince the sheath will need to expand to a greater outer diameter thanwill the core.

Delamination may also occur due to mismatches between the thermalshrinkage that occurs after the pyrolysis temperature reaches thetemperature at which one or more polymers begin to thermally degrade. Inorder to inhibit or virtually eliminate this different cause ofdelamination, the amount of anti-substructure collapse particle loadingmay be adjusted in the core to reduce the amount of shrinkage that mayoccur in the core. In other words, as more and more of the volume of thecore is taken up by the particles instead of by polymer, shrinkage ofthe core due to the core polymer flowing/shrinking/thermally degradingis reduced more and more. Thus, the wt % of the anti-substructurecollapse particles in the core composition is selected such that thepolymeric sheath material shrinks along a length of the fiber no morethan +/−15% than that of the polymeric core material, but in any case isat least 5 wt %.

An alternative technique for inhibiting or virtually eliminating thissecond cause of delamination, the polymeric sheath material may be afirst polymer exhibiting a first coefficient of thermal shrinkage abovea temperature at which the first polymer starts to thermally degrade,the polymeric core material is a second polymer having a secondcoefficient of thermal shrinkage above a temperature at which the secondpolymer starts to thermally degrade, and the first and secondcoefficients of thermal shrinkage differ from one another by no morethan 15%.

The polymeric core material may be any polymer or copolymer known in thefield of membrane fluid separation. Suitable polymeric core materialsinclude a polyaramide available as NOMEX that consists of repeatingunits of diamino mesitylene isophthalic acid, Ultem as described above,and polybenzimidazole (FBI). Since the main purpose of the polymericcore material is to provide strength, it desirably exhibits a relativelyhigh tensile strength. As a rough proxy for tensile strength, thepolymer core material's glass transition may be utilized. Thus, thepolymeric core material may have a glass transition temperature equal toor greater than 200° C., typically equal to or greater than 280° C.

Suitable solvents for the core and dope solution polymer(s) may include,for example, dichloromethane, tetrahydrofuran (THF),N-methyl-2-pyrrolidone (NMP), and others in which the resin issubstantially soluble, and combinations thereof. For purposes herein,“substantially soluble” means that at least 98 wt % of the polymer inthe solution is solubilized in the solvent. Typical solvents includeN-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC),N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO),gamma-butyrolactone (BLO), dichloromethane, THF, glycol ethers oresters, and mixtures thereof. The core dope solution may also include apore former such as CaBr₂.

The concentration(s) of the polymer(s) in the core and dope solutions istypically driven by the configuration of the precursor compositemembrane (the green fiber before pyrolysis). Typically, theconcentration will range from 12-35 wt % (or optionally 15-30 wt % oreven 18-22 wt %).

The precursor composite hollow fibers are then at least partially, andoptionally fully, pyrolyzed to form the final CMS membrane. Because ofthe presence of the silica particles inside the pores of the precursorfiber core, those pores do not collapse during pyrolysis as theyordinarily would in conventional CMS membrane manufacturing processes.After pyrolysis, the silica particle-filled pores form aninterconnecting network in the core through which a high flux of gas isallowed. If the silica particles were not present, the pores of theprecursor fibers would collapse during pyrolysis to yield an effectivelythick dense film. Since the flux is related to the thickness of thedense film, the flux in the absence of the silica particles would beundesirably low.

While any known device for pyrolyzing the membrane may be used,typically, the pyrolysis equipment includes a quartz tube within afurnace whose temperature is controlled with a temperature controller.

Pyrolysis may be optionally carried out under vacuum typically rangingfrom about 0.01 mm Hg to about 0.10 mm Hg or even as low as 0.05 mm Hgor lower. In this case, the ends of the quartz tube are sealed in orderto reduce any leaks. In vacuum pyrolysis, a vacuum pump is used inconjunction with a liquid nitrogen trap to prevent any back diffusion ofoil vapor from the pump and also a pressure transducer for monitoringthe level of vacuum within the quartz tube.

Typically, the pyrolysis atmosphere inside the chamber is an inert gashaving a relatively low concentration of oxygen, such as those disclosedby US 2011/0100211. By selecting a particular oxygen concentration(i.e., through selection of an appropriate low-oxygen inert purge gas)or by controlling the oxygen concentration of the pyrolysis atmosphere,the gas separation performance properties of the resulting CMS membranemay be controlled or tuned. While any inert gas in the field ofpolymeric pyrolysis may be utilized as a purge gas during pyrolysis,suitable inert gases include argon, nitrogen, helium, and mixturesthereof. The ambient atmosphere surrounding the CMS membrane may bepurged with an amount of inert purge gas sufficient to achieve thedesired oxygen concentration or the pyrolysis chamber may instead becontinuously purged. While the oxygen concentration, either of theambient atmosphere surrounding the CMS membrane in the pyrolysis chamberor in the inert gas is less than about 50 ppm, it is typically less than40 ppm or even as low as about 8 ppm, 7 ppm, or 4 ppm.

While the pyrolysis temperature may range from 500-1,000° C., typicallyit is between about 450-800° C. As two particular examples, thepyrolysis temperature may be 1,000° C. or more or it may be maintainedbetween about 500-550° C. The pyrolysis includes at least one ramp stepwhereby the temperature is raised over a period of time from an initialtemperature to a predetermined temperature at which the polymer ispyrolyzed and carbonized. The ramp rate may be constant or follow acurve. The pyrolysis may optionally include one or more pyrolysis soaksteps (i.e., the pyrolysis temperature may be maintained at a particularlevel for a set period of time) in which case the soak period istypically between about 1-10 hours or optionally from about 2-8 or 4-6hours.

An illustrative heating protocol may include starting at a first setpoint (i.e., the initial temperature) of about 50° C., then heating to asecond set point of about 250° C. at a rate of about 3.3° C. per minute,then heating to a third set point of about 535° C. at a rate of about3.85° C. per minute, and then a fourth set point of about 550° C. at arate of about 0.25 degrees centigrade per minute. The fourth set pointis then optionally maintained for the determined soak time. After theheating cycle is complete, the system is typically allowed to cool whilestill under vacuum or in the controlled atmosphere provided by purgingwith the low oxygen inert purge gas.

Another illustrative heating protocol (for final temperatures up to 550°C. has the following sequence: 1) ramp rate of 13.3° C./min from 50° C.to 250° C.; 2) ramp rate of 3.85° C./min from 250° C. to 15° C. belowthe final temperature (T_(max)); 3) ramp rate of 0.25° C./min fromT_(max)-15° C. to T_(max); 4) soak for 2 h at T_(max).

Yet another illustrative heating protocol (for final temperatures ofgreater than 550° C. and no more than 800° C. has the followingsequence: 1) ramp rate of 13.3° C./min from 50° C. to 250° C.; 2) ramprate of 0.25° C./min from 250° C. to 535° C.; 3) ramp rate of 3.85°C./min from 535° C. to 550° C.; 4) ramp rate of 3.85° C./min from 550°C. to 15° C. below the final temperature T_(max); 5) ramp rate of 0.25°C./min from 15° C. below the final temperature T_(max) to T_(max); 6)soak for 2 h at T_(max).

Still another heating protocol is disclosed by U.S. Pat. No. 6,565,631.Its disclosure is incorporated herein by reference.

After the heating protocol is complete, the membrane is allowed to coolin place to at least 40° C. while still under vacuum or in the inert gasenvironment.

While the source of inert gas may already have been doped with oxygen toachieve a predetermined oxygen concentration, an oxygen-containing gassuch as air or pure oxygen may be added to a line extending between thesource of inert gas and the furnace via a valve such as a micro needlevalve. In this manner, the oxygen-containing gas can be added directlyto the flow of inert gas to the quartz tube. The flow rate of the gasmay be controlled with a mass flow controller and optionally confirmedwith a bubble flow meter before and after each pyrolysis process. Anyoxygen analyzer suitable for measuring relatively low oxygenconcentrations may be integrated with the system to monitor the oxygenconcentration in the quartz tube and/or the furnace during the pyrolysisprocess. Between pyrolysis processes, the quartz tube and plate mayoptionally be rinsed with acetone and baked in air at 800° C. to removeany deposited materials which could affect consecutive pyrolyses.

Following the pyrolysis step and allowing for any sufficient cooling,the gas separation module is assembled. The final membrane separationunit can comprise one or more membrane modules. These can be housedindividually in pressure vessels or multiple modules can be mountedtogether in a common housing of appropriate diameter and length. Asuitable number of pyrolyzed fibers are bundled to form a separationunit and are typically potted with a thermosetting resin within acylindrical housing and cured to form a tubesheet. The number of fibersbundled together will depend on fiber diameters, lengths, and on desiredthroughput, equipment costs, and other engineering considerationsunderstood by those of ordinary skill in the art. The fibers may be heldtogether by any means known in the field. This assembly is typicallydisposed inside a pressure vessel such that one end of the fiberassembly extends to one end of the pressure vessel and the opposite endof the fiber assembly extends to the opposite end of the pressurevessel. The tubesheet and fiber assembly is then fixably or removablyaffixed to the pressure vessel by any conventional method to form apressure tight seal.

For industrial use, a permeation cell or module made using the pyrolyzedCMS membrane fibers may be operated, as described in U.S. Pat. No.6,565,631, e.g., as a shell-tube heat exchanger, where the feed ispassed to either the shell or tube side at one end of the assembly andthe product is removed from the other end. For maximizing high pressureperformance, the feed is advantageously fed to the shell side of theassembly at a pressure of greater than about 10 bar, and alternativelyat a pressure of greater than about 40 bar. The feed may be any gashaving a component to be separated, such as a natural gas feedcontaining an acid gas such as CO₂ or air or a mixture of an olefin andparaffin.

The described preparation of CMS membranes leads to an almost purecarbon material in the ultrathin dense film. Such materials are believedto have a highly aromatic structure comprising disordered sp² hybridizedcarbon sheet, a so-called “turbostratic” structure. The structure can beenvisioned to comprise roughly parallel layers of condensed hexagonalrings with no long range three-dimensional crystalline order. Pores areformed from packing imperfections between microcrystalline regions inthe material and their structure in CMS membranes is known to beslit-like. The CMS membrane typically exhibits a bimodal pore sizedistribution of micropores and ultramicropores—a morphology which isknown to be responsible for the molecular sieving gas separationprocess.

The micropores are believed to provide adsorption sites, andultramicropores are believed to act as molecular sieve sites. Theultramicropores are believed to be created at “kinks” in the carbonsheet, or from the edge of a carbon sheet. These sites have morereactive unpaired sigma electrons prone to oxidation than other sites inthe membrane. Based on this fact, it is believed that by tuning theamount of oxygen exposure, the size of selective pore windows can betuned. It is also believed that tuning oxygen exposure results in oxygenchemisorption process on the edge of the selective pore windows. US2011/0100211 discloses typical conditions for tuning the amount ofoxygen exposure. The pyrolysis temperature can also be tuned inconjunction with tuning the amount of oxygen exposure. It is believedthat lowering pyrolysis temperature produces a more open CMS structure.This can, therefore, make the doping process more effective in terms ofincreasing selectivity for challenging gas separations for intrinsicallypermeable polymer precursors. Therefore, by controlling the pyrolysistemperature and the concentration of oxygen one can tune oxygen dopingand, therefore, gas separation performance. In general, more oxygen andhigher temperature leads to smaller pores. Higher temperatures generallycause the formation of smaller micro and ultramicropores, while moreoxygen generally causes the formation of small selective ultramicroporeswithout having a significant impact on the larger micropores into whichgases are absorbed.

EXAMPLES Comparative Example

Precursor monolithic composite fibers were spun from a spinneret from asingle dope solution. The fibers are monolithic in the sense that thereis no sheath/core composite structure. In other words, the dope solutionwas fed from a single annulus surrounding the bore fluid. The dopesolution included wt % 6FDA:BPDA/DAM dissolved in wt % NMP.

The bore fluid and dope solution were fed to the spinneret at a rate of1 cc/min and 3 cc/min, respectively at a spin temperature of C. Thenascent fibers were passed through an air gap of 16 cm and coagulated ina water coagulant (quench) bath at a temperature of 38° C. The solidfibers were wound onto a take-up roll at rate of 15 m/min.

The resultant composite precursor hollow fibers were pyrolyzed in a 78.9mm diameter tube furnace as follows. Beginning at room temperature, thefurnace temperature was ramped (increased) at a rate of 13.3° C./min upto 250° C., ramped a rate of 3.8 C/min up to 535° C., ramped at a rateof 0.2 C/min up to 550° C., and maintained at 550° C. for 1.75 hours.The pyrolysis atmosphere was a mixture of 30 ppm O₂ in Argon fed to thetube furnace at a flow rate of 380 cc/min.

Example

Precursor composite hollow fibers were spun with a double spinneret fromcore and sheath dope solutions. The core dope solution included 22 wt %6FDA:BPDA/DAM dissolved in 78 wt % NMP. The sheath dope solutionincluded 5.5 wt % CaBr₂, 4% silica, and 16 wt % 6FDA:BPDA/DAM dissolvedin 74.5 wt % NMP. The silica particles were obtained from SpectrumChemical Corp. under the trade name Cab-O-Sil M-5. The bore fluid was amixture of 85 wt % NMP and 15 wt % H₂O.

The bore fluid, the core dope solution, and the sheath dope solutionwere fed to the spinneret at a rate of 90 cc/hr, 200 cc/hr, and 40cc/hr, respectively at a spin temperature of 79° C. The nascent fiberswere passed through an air gap of 16 cm and coagulated in a watercoagulant (quench) bath at a temperature of 38° C. The solid fibers werewound onto a take-up roll at rate of 15 m/min.

The resultant composite precursor hollow fibers were pyrolyzed in a 78.9mm diameter tube furnace as follows. Beginning at room temperature, thefurnace temperature was ramped (increased) at a rate of 13.3° C./min upto 250° C., ramped a rate of 3.8 C/min up to 535° C., ramped at a rateof 0.2 C/min up to 550° C., and maintained at 550° C. for 1.75 hours.The pyrolysis atmosphere was a mixture of 30 ppm O₂ in Argon fed to thetube furnace at a flow rate of 380 cc/min.

Separation Characteristics:

The pyrolyzed fibers were then tested for CO₂ permeance and CO₂/CH₄selectivity. The results are shown in Table I below.

TABLE 1 Separation characteristics of CMS membranes. CO₂ Permeance(GPUs) CO₂/CH₄ selectivity Comparative example 244 ± 53  40 ± 8 Example538 ± 116 45 ± 6

As seen in Table I, by spinning the fiber with a composite structure andby including silica particles in the core dope solution, a 12.5%increase in CO₂/CH₄ selectivity and a 120% increase in CO2 permeance maybe realized in comparison to conventional monolithic fibers not havingsilica particles in the core dope solution. Taking the Backgrounddiscussion into consideration, this tends to show that the problem oflow permeance exhibited by conventional CMS membranes has been solved.

The advantages of producing CMS membranes from composite hollow fibershaving sub micron SiO₂ particles in the core dope solution may also beseen in a comparison of FIGS. 2 and 3. As seen in FIG. 2, the coresubstructure of a monolithic fiber without silica particles collapses toresult in a very thick dense separation layer. Recall that thickseparation layers exhibit very low flux since flux is directly relatedto the separation layer thickness. As seen in FIG. 3, however, twodistinct layers are seen. The sheath layer (which provides most of theseparation) is thin and dense. The core layer is porous. This tends toshow that the presence of silica in the core dope solution prevents coresubstructure collapse during pyrolysis and maintains porosity in thecore substructure. Finally, it may be said that the sheath layer has avery small thickness. The permeability of a CMS membrane made from6FDA:BPDA/DAM is 2300 Barrers. So, 538 GPUs measured for composite fiberwill correspond to skin thickness of only 4 microns.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing i.e.anything else may be additionally included and remain within the scopeof “comprising.” “Comprising” is defined herein as necessarilyencompassing the more limited transitional terms “consisting essentiallyof” and “consisting of”; “comprising” may therefore be replaced by“consisting essentially of” or “consisting of” and remain within theexpressly defined scope of “comprising”.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

What is claimed is:
 1. A method for producing a CMS membrane fiber,comprising the steps of: forming a composite precursor polymeric hollowfiber having a sheath covering a hollow core, the core being solidifiedfrom a core composition comprising a polymeric core material dissolvedin a core solvent and anti-substructure collapse particles insoluble inthe core solvent, the anti-substructure collapse particles beingdisposed within pores formed in the polymeric core material, the sheathbeing solidified from a sheath composition comprising a polymeric sheathmaterial dissolved in a sheath solvent, the anti-substructure collapseparticles having an average size of less than one micron; and pyrolyzingthe composite precursor polymeric hollow fiber up to a peak pyrolysistemperature T_(P), wherein the anti-substructure collapse particles aremade of a material or materials that either: i) have a glass transitiontemperature T_(G) higher than T_(P), ii) have a melting point higherthan T_(P), or ii) are completely thermally decomposed during saidpyrolysis step at a temperature less than T_(P).
 2. The method of claim1, wherein the material or materials of the anti-substructure collapseparticles are selected from the group consisting of: polymer, glasses,ceramics, graphite, silica and mixtures of two or more thereof.
 3. Themethod of claim 2, wherein the material of the anti-substructurecollapse particles is polybenzimidazole.
 4. The method of claim 2,wherein the material of the anti-substructure collapse particles issilica.
 5. The method of claim 1, wherein the material or materials ofthe anti-substructure collapse particles are selected from cellulosicmaterials and polyethylene.
 6. The method of claim 1, wherein thepolymeric sheath material and the polymeric core material are a samepolymer or copolymer.
 7. The method of claim 6, wherein a wt % of thepolymer or copolymer in the core composition is lower than a wt % of thepolymer or copolymer in the sheath composition.
 8. The method of claim1, wherein the polymeric sheath material is different from the polymericcore material.
 9. The method of claim 8, wherein the polymeric sheathmaterial comprises a major amount of a first polymer or copolymer and aminor amount of second polymer or copolymer and the polymeric corematerial comprises a minor amount of the first polymer or copolymer anda major amount of the second polymer or copolymer.
 10. The method ofclaim 8, wherein the polymeric sheath material is a first polymer havinga first coefficient of thermal expansion, the polymeric core material isa second polymer having a second coefficient of thermal expansion, andthe first and second coefficients of thermal expansion differ from oneanother by no more than 15%.
 11. The method of claim 10, wherein firstcoefficient of thermal expansion is greater than the second coefficientof thermal expansion.
 12. The method of claim 10, wherein a wt % of theanti-substructure collapse particles in the core composition is selectedsuch that the polymeric sheath material shrinks along a length of thefiber no more than +1-15% than that of the polymeric core material, butin any case is at least 5 wt %.
 13. The method of claim 8, wherein thepolymeric sheath material is a first polymer exhibiting a firstcoefficient of thermal shrinkage above a temperature at which the firstpolymer starts to thermally degrade, the polymeric core material is asecond polymer having a second coefficient of thermal shrinkage above atemperature at which the second polymer starts to thermally degrade, andthe first and second coefficients of thermal shrinkage differ from oneanother by no more than 15%.
 14. The method of claim 8, wherein thepolymeric sheath material is a first polymer, the polymeric corematerial is a second polymer, and the second polymer has a glasstransition temperature equal to or greater than 200° C.
 15. The methodof claim 14, wherein the second polymer has a glass transitiontemperature equal to or greater than 280° C.
 16. The method of claim 1,wherein the polymeric core material is a polyaramide consisting ofrepeating units of diamino mesitylene isophthalic acid.
 17. The methodof claim 1, wherein the polymeric core material is the condensationproduct of 2,2-bis[4-(2,3-dicarboxyphenoxy)phenyl]propane dianhydrideand m-phenylenediamine or p-phenylenediamine.
 18. The method of claim 1,wherein the polymeric core material is polybenzimidazole
 19. The methodof claim 1, wherein each of the polymeric core and sheath materials ismade of a polymer or copolymer independently selected from the groupconsisting of polyimides, polyether imides, polyamide imides, celluloseacetate, polyphenylene oxide, polyacrylonitrile, and combinations of twoor more thereof.
 20. The method of claim 19, wherein the polymericsheath material is made of a polyimide.
 21. The method of claim 20,wherein the polyimide consists of the repeating units of formula I:


22. The method of claim 20, wherein the polyimide is 6FDA:BPDA/DAM. 23.The method of claim 20, wherein the polyimide is selected from the groupconsisting of: 6FDA:mPDA/DABA and 6FDA:DETDA/DABA.
 24. The method ofclaim 19, wherein the polymeric sheath material is poly(4,4′-oxydiphenylene-pyromellitimide).
 25. The method of claim 19,wherein the polymeric sheath material consists of the repeating units offormulae II and III:


26. The method of claim 20, wherein the polyimide consists of repeatingunits of formula IV:


27. A CMS membrane fiber produced according to the method of claim 1.28. A CMS membrane module including a plurality of the CMS membranefibers of claim
 27. 29. A method for separating a gas mixture,comprising the steps of feeding a gas mixture to the CMS membrane moduleof claim 28, withdrawing a permeate gas from the CMS membrane modulethat is enriched in at least one gas relative to the gas mixture, andwithdrawing a non-permeate gas from the CMS membrane module that isdeficient in said at least one gas relative to the gas mixture.